It is well known that internal combustion engines embodied as piston engines cannot aspirate the air quantities required for combustion in their cylinders uniformly through the intake tube, but in fact can only do so nonuniformly, because during the compression, combustion and expulsion strokes (in this case, referring to a four-stroke engine) the inlet valve remains closed, so that the transportation of the air quantities through the intake tube is interrupted. Depending on the number of cylinders of the engine, pulsations or fluctuations occur in the air column in the intake tube. Not infrequently, these fluctuations have a positive supercharging effect, for which the air intake system, and the exhaust manifold adapted to it, may even be intentionally designed; nevertheless, these fluctuations necessarily become a problem if, superimposed on the intake process, they cause an inaccurate signal to be furnished by measuring devices which are supposed to ascertain the air throughput through the intake tube.
Since the fuel must be distributed to engines by a mixture former in proportion with the quantity of air aspirated, measuring or determining the aspirated air quantity is necessary in order finally to obtain an electrical or mechanical indication of the quantity of air aspirated, or at least required, by the engine at a given moment. In the connection, mixture formers such as fuel injection systems and the like are known, which ascertain the aspirated air quantity by measuring the pressure in the intake tube or use air flow rate meters primarily comprising mechanical components, which for instance control a quantity distributor for the fuel metering via a baffle plate articulatedly supported in the intake tube, by means of the baffle plate deflection effected under the influence of the air masses flowing past it (Bosch K-Jetronic).
The following discussion of the subject of air flow rate measurement, taking into account air column fluctuations in the intake tube, encompasses the disposition of a hot wire flow rate meter in the intake tube which furnishes an indication, or reading, that is as closely proportional as possible to the aspirated air quantity. It is to be understood, however, that this discussion is equally valid for any other air flow rate meters with which similar problems arise because of the fluctuation of the air column in the intake tube. Specifically, if the air flow rate meter uses a hot wire, then the aspirated air column, oscillating back and forth, leads to an inaccurate air throughput reading, because the hot wire flow rate meter is incapable of distinguishing between air masses which flow from the intake tube inlet toward the engine and masses which undergo a movement impetus in the opposite direction because of a brief reverse flow or pulsation. The necessary, system-dictated result, caused by this fluctuation or oscillation of the air column back and forth, is that a multiple reading is produced for one and the same mass of air. Thus an incorrect fuel enrichment cannot be entirely precluded, at least for specific operating conditions such as full load at low rpm, because a greater air throughout may be indicated than actually exists. This may cause tuning problems in the entire system.